This invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium and which is excellent in runnabilities and durability without deteriorating its magnetic properties, and is suitable for high-density recording.
The magnetic recording media can be employed for audio tapes, video tapes, computer tapes, floppy disks and the like, but in recent years, magnetic recording tapes for high-difinition VTR are being developed.
Of the above mentioned magnetic recording media, the computer tapes, floppy disks and such magnetic recording tapes are exposed to severer use conditions as compared with the audio tapes and the video tapes, and they are thus required to have a high reliability, i.e., excellent runnabilities and durability.
Heretofore, in order to improve the above mentioned runnabilities and durability, it has been carried out to heighten a smoothness of the surfaces of the media and to prevent them from charging by sliding friction. That is, the runnabilities and the durability have been improved by adding a great deal of a conductive carbon (for example, in an amount of 5 to 7 parts by weight based on 100 parts by weight of a magnetic powder) or lubricants to a magnetic layer of each medium.
However, the addition of much carbon or lubricant just described, in particular, the addition of much carbon, loweres its recording density. That is, in the magnetic recording media for high-density recording, a ununiformly dispersed magnetic powder in the magnetic layer is necessary, but the addition of much carbon will lead to the deterioration in this dispersion and the reagglomeration of the magnetic powder, with the result that the prepared media will be diminished in magnetic properties.
Accordingly, method of overcoming such a problem of the conventional technique have been disclosed in, for example, Japanese Provisional Patent Publication No. 114206/1975 and No. 19712/1979. They are (1) a method for subjecting a support of the magnetic recording medium to a conductive treatment with the intention of cancelling the addition of a conductive material such as carbon to the magnetic layer, and (2) a method for disposing, on a conductively treated support, a magnetic layer in which an alkyl sulfate or anhydrous calcium chloride is contained, in which case, conductivity is given to the magnetic layer by means of an ion conduction of such a compound.
In these methods, however, much attention is paid to the point of a charge prevention of the medium. Therefore, under the conditions where the charge is brought about, such methods are effective to some extent but cannot give a sufficient charge prevention effect to the media for the floppy disks, the highly accurate magnetic tapes and the like which will be employed under severe conditions and in which the great durability is thus required. The latter method (2) is larger in the charge prevention effect than the former method (1), but in the latter case (2) of adding the alkyl sulfate or the like to the magnetic layer so as to provide the magnetic layer with the conductivity by the ion conduction effect cannot be obtained under the condition of a low humidity.
Further, in the magnetic recording medium in which there is used, as the magnetic powder, a hexagonal system magnetic powder having a higher insulating properties than Co-containing .gamma.-Fe.sub.2 O.sub.3 and suitable for high-density recording, any satisfactory durability cannot be obtained only by subjecting the support to the conductive treatment, since its conductivity might be insufficient. When the support to the conductive treatment has not been done is used, a large amount of the conductive carbon must be added to the magnetic layer disadvantageously. In this case, the dispersion of the magnetic powder will deteriorate, and the magnetic properties of the medium will become poor owing to the use of much carbon. After all, it will be meaningless inconveniently to make use of the hexagonal system magnetic powder suitable for high-density recording and having the high insulation resistance.
Moreover, when a metallic powder is used as the conductive material in the conductive layer, peeling strength of the magnetic recording medium will diminish, and costs will be high disadvantageously.